The feasibility of optical monitoring of larval salmon lice

Salmon lice are a persistent challenge in salmon farming and have major economic and environmental implications. The lack of efficient ways to monitor the abundance and distribution of salmon lice larvae in the water in and around salmon farms is limiting the prediction of the timing of infestation necessary for better treatment strategies. The goal of this thesis was to develop and test an optical detector prototype for monitoring and identification of salmon lice larvae. A pilot study was conducted to investigate a range of different optical properties of salmon lice larvae, Lepeophtheirus salmonis and another relevant test species, Acartia tonsa. The properties investigated were extinction, polarization, movement dynamics and autofluorescence. The study showed a significant difference in autofluorescence between the two species, with A. tonsa exhibiting a strong gut fluorescence above 600 nm, which was absent in salmon lice. A microspectroscopy setup with simultaneous auto fluorescence imaging and spectroscopy was conducted to investigate the spectral components of the autofluorescence from salmon lice and other zooplankton, and it was found that chlorophyll autofluorescence was solely present in algae-eating zooplankton, while all species exhibited a broad cyan fluorescence signature. Using these results a dual-band fluorescence detector was built, measuring autofluorescence of zooplankton in the wavelength bands of 500-550 nm and 675-725 nm induced by a 41 0nm laser illumination. Using this detector it was shown that non-eating zoo plankton, including salmon lice, could be distinguished from algae-eating zooplankton with a sensitivity and specificity of respectively 98.9 % and 94.1 % in laboratory conditions. Finally the signal-to-noise ratio of the detection system was investigated in experiments that attempt to simulate field conditions at salmon farms. It was seen that the signal-to-noise ratio was negatively impacted by several factors, like the background fluorescence of fjord water. Future work will thus focus on how to overcome this degeneration of the signal-to-noise ratio when monitoring in field conditions.